I'm aware that this has been asked before, but as a considerable period of time has passed, it might be worth reviewing, especially as there are now a plethora of Area 51 originated sites which would benefit from growth in their user base.

There have been ocassions where I've pointed people towards StackExchange sites and it may well encourage others to do the same if they could say:

Go to http://sitename.stackexchange.com/refer/7872, create yourself an account and ask your question because you'll almost certainly get a fantastic answer to your question there. Even if you don't right away, you will eventually and either way, you'll probably find yourself visiting the site daily to add to it!

(or something like that, anyway! :)

I'd like to propose a couple of badges:

BRONZE BADGE; Referred a user who has completed their profile and asked a question which has received X (2? - ensures that the badge can't be "gamed" by the referrer up-voting the referee's question) up votes.

SILVER BADGE; Referred a user who has asked X (2?) questions, given Y (5?) answers and received Z up-votes (10?)

Note: The definitions I've given for the badges are entirely speculative and I look forward to up-voting any suggestions, in answers, that are better than mine. I don't doubt there will be many that are better =)